1. Technical Field
The present invention relates to an optical measurement apparatus and an optical measurement method.
2. Background Art
In recent years, attention is paid to an optical coherence tomography (OCT) to acquire an image reflecting a surface structure or an internal structure of a measurement object by using light (JP-A-2011-218155 (Patent Literature 1)). Since the OCT is non-invasive to the human body, its application to, especially, medical fields and biological fields is expected. In the ophthalmological field, an apparatus to form an image of an eyeground or a cornea is put in practical use. In the OCT, light from a light source is branched into signal light which is irradiated to a measurement object and reference light which is not irradiated to the measurement object but is reflected by a reference light mirror, and a signal is obtained by combining and interfering the signal light reflected from the measurement object and the reference light.
The OCT is roughly classified into a time domain OCT and a Fourier domain OCT according to a scanning method (hereinafter referred to as z-scan) in an optical axis direction of measurement position. In the time domain OCT, a low-coherence light source is used as the light source, and the z-scan is performed by scanning the reference light mirror at the time of measurement. By this, only a component included in the signal light, whose light path length is coincident with that of the reference light, interferes, and a desired signal is demodulated by performing envelope detection on the obtained interference signal.
On the other hand, the Fourier domain OCT is further classified into a wavelength scanning type OCT and a spectrum domain OCT. In the wavelength scanning type OCT, a wavelength scanning type light source capable of scanning the wavelength of emitted light is used, and the z-scan is performed by scanning the wavelength at the time of measurement. A desired signal is demodulated by Fourier transforming the wavelength dependence (interference spectrum) of detected interference light intensity.
In the spectrum domain OCT, a wide-band light source is used as the light source. Generated interference light is spectrally dispersed by a spectroscope, and the interference light intensity (interference spectrum) of each wavelength component is detected, which corresponds to performing the z-scan. A desired signal is demodulated by Fourier transforming the obtained interference spectrum.
Incidentally, as disclosed in JP-A-2010-169503 (Patent Literature 2), in the OCT for the most common eye fundus diagnosis apparatus, an NA of approximately 0.02 is general.
In the related art OCT apparatus described above, there are following problems. In the time domain OCT, the mirror is required to be mechanically scanned at high speed in order to perform the z-scan, and there is a problem that when an angle shift occurs in the optical axis of the reference light by the scanning of the mirror, the signal intensity decreases, and consequently, the SN ratio decreases. In the Fourier domain OCT, the light source to sweep the wavelength at high speed or the spectroscope is required in order to perform the z-scan, and there is problem that both of them are expensive and large, and the OCT apparatus itself becomes expensive and large.
Then, the inventor et al. investigated a structure in which as shown in FIG. 1, a laser light source having a long coherence length is used as a light source, laser light (signal light) is condensed and irradiated on a measurement object by using a high NA objective lens, and a condensing position is scanned by scanning the objective lens, so that a tomographic image of the measurement object is acquired. In the OCT apparatus of this structure, three-dimensional measurement is enabled by separating signals in the optical axis direction by using the principle that a reflected light component included in the signal light, which is from a portion other than the focal point of the objective lens, is different from the reference light in wavefront curvature and does not interfere therewith. This OCT apparatus is basically different in principle from the related art OCT apparatus which uses the wide-band light source (low-coherence light source) or the wavelength scanning type light source. In this structure, since the large and expensive wavelength scanning type light source or spectroscope is not required, the small and inexpensive OCT apparatus can be provided. However, since the laser light having a long coherence length is used, there is a problem that image deterioration due to optical interference such as inter-layer crosstalk or speckle is liable to occur, and a bad effect is exerted on an acquired tomographic image.